Electrical wave filter



L 1 1 Jan. 25,1927. J. ZOBEL ,6 5,252

ELECTRICAL WAVE FILTER File d June 9, 1923 3 Sheets-Sheet 1 i fi z WWW\/\/ IW g F 6 I 1 fi z y WVWW A INVENTOR C mORNE Y 1 i 2 2 Jan. 25,1927. 0' J. ZOBEL ,6 5

ELECTRICAL WAVE FILTER Filed June 9, 1923 3 Sheets-Sheet 2 i 9 5 INVENTOR ATTORNEY O. J. ZOBEL Jan. 25, 1927. 1,615,252

ELECTRI CAL WAVE FILTER Filed June 9, 1925 3 SheetsSheet 5 Side (Zia-a0? Kwclslance ii'equency cycles per second IN VEN TOR MTORNEY Patented Jan. 25, 1927.

UNITED} STATES PATENT OFFICE.

OTTO J'. ZOBEL, OF YORK, N. Y., ASSIGNOR TO AMERTCAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

ELECTRICAL WAVE FILTER.

Application filed June 9,

A principal object of my invention is to provide an electrical wave-filterof the type having recurrent sections that shall simulate the impedance of a transmission line over 5 a Wide frequency range including low frequencies, Another object of my invention is to improve the characteristic impedance of a wave-filter by the introduction of resistances with its other impedance elements.

[ Still another object of my invention. is to provide a wave-filter of favorable charac teristics for use in connection with phantom and side circuits. These and various other objects of my invention will be made ap- 1 parent in the following specification and claims taken with the accompanying drawings, in which I have disclosed a single specific embodiment of the invention by way of illustration of the general principles involved. The following specification will re late to this particular example of the invention, leaving the invention to be defined in the appended claims.

Referring to the drawings, Figure 1 is a 2 diagram showing a high pass filter and a .low pass filter in respective parallel branches in a transmission line; Fig. 2 is a corresponding diagram showing the elements of the wave-filters more spec fically; Fig. 3 is another diagram showing the terminal ele-- ments of the wave-filters; Fig. 4 is a diagram showing my invention embodied in a pair of side circuits and a phantom circuit; Figs. 5 and 6 are simplified diagrams 36 correspondin to Fig. 4, which will be utilized 1n. exp aining the theory of Fig. 4; Fig. 7 is a diagram giving impedance-frequency characteristics for the side and phantom circuits of Fig. 4. V,

I will illustrate the principles involved in my invention by giving a specific example of design such as might easily arise in prac-' tical engineering work.

In an extended transmission line Z, shown inFig. 1, carrying currents of various frequencies, high and low, it is desired to provide parallel branches at a certain point,

such that the high frequencies shall be transmitted through one branch and the low frequencies through the other branch. In the particular case under consideration, the critical dividing frequency is assumed to be at 3,000 cycles per second. It is well known- 1923. Serial No. 644,458.

that high frequencies are attenuated more than low frequencies and one purpose of the separation of the currents into the two channels, as indicated, is to. provide amphfication by means of a repeater in the high pass branch. I

The line Z has a characteristic impedance of 620 ohms resistance at high frequency, but at 135 cycles per second its characteristic impedance is given by the complex number 720-i365.

The general design for appropriate high pass and low pass filters is shown in Fig. 2, where the high pass filter is shown above and'the low pass filter below. In accordance with well-known properties of such filters, we have the following equations, where f, is the critical or cut-off frequency:

At high frequencies, the mid-series characteristic-impedance or the mid-shunt characteristic impedance of the high pass wavefilter reduces to the expression 1 '1 '2; Where 2, is one of the series impedance ele ments and .2 is one of the shunt impedance elements. Accordingly we have as another equation of design The two wave-filters, one high pass and the other low pass, must be complementaryfto' each other so that as nearly as practicable, at frequencies above or below the critical frequency, each one will be a reactanceannulling shunt for the other. The complementary relationshipbetween the two filters requires that where a is a series impedance element of the low pass filter and 2 is a shunt impedance element of the low pass filter.

The foregoing equations (1), (2), (3) and (4) are sufficient to determine the capacities and inductances designated'on Fig. 2

by the characters C L L and 6,. Their values work out as follows:

It can be shown that the complementary high pass wave-filter of Fig. 3 aflords approximately such a shunt for the low pass wave-filter; indeed the w-series ending with oa -0.809 is the optimum ending to make each wave-filter of Fig. 3 as nearly as practicable the best reactance annulling shunt for the other.

The impedance of the low pass wave-filter with w-series' endin and with an ideal reactance annulling shunt is and in the particular case under consideration w=0.809. Accordingly equation (6) is 1 an approximate expression for the impedance of the combined high pass filter and low pass filter as shown in Fig. 3, for frequencies transmitted by the low pass wavefilter.

At high frequency, the impedance of the low pass filter with m-series ending approaches infinity and the impedance of the high pass filter with w-series ending becomes substantially the same as its mid-series characteristic impedance which welhave already taken as 620 ohmsv resistance in equation (3), the same as the high frequency impedance of the line Z.

But at a frequency'of 135 cycles per second, the impedance of the line Z is known to be' Giving to z, the value afiorded at l35 cycles by the condenser C,, it works out that z (135)=-i6880. (8') Applying this value in the equation ob- This gives the 'Fig.

tained by equating (6) and (7) and also giving to w its value 0.809, the result is obtained that R,=76.08 ohms. (10) The addition of this resistance in the series elements of the low pass wave-filter will not affect the impedance at high' frequency because then the impedance of the low pass filter approaches infinity in any case.

In Fig. 4, Z and Z represent two side circuits comprised in a phantom circuit and it is desired first to provide high pass and low pass branches in each side circuit in accordance with the foregoing development and further to make whatevermodification ma be necessary to guard against reflection e fects in the phantom circuit. We shall first give attention to the specific design of the side circuit Z. I

The repeater R is interposed in the high pass branch and on each side of it there is a high pass wave-filter consistingof two complete mid-series sections each with its terminal series impedance extended enough to give the 0.809-series ending. With this ending, the low pass filter gives approximately the proper reactance annulling shunt at the end of each high pass filter distant from the repeater R adjacent to the repeater R the more exact reactance annulling shunt is provided with the elements designated The series impedance elements in Figs. 1, i

2 and 3 have been shown on only one side of th line, but for line balance they should be di posed equally on the two sides of the line and this has been done throughout in 4. 'For example, two condensers 1n Fig. 4 each of capacity value 2C are equivalent to the single condenser C in Fig. 3. i The low pass wave-filter is made with two complete mid-series sections designated S and S. The junction points 1 and 2 between these sections are midpoints of the low pass filter at which it can be opened, if that should-be desired, for testing purposes or at which testing apparatus can be bridged across. The series inductance windings are on the same core "and this is intended to be m ings taken together. Thus I section.

indicated byv the dotted lines connecting the adjacent arrowheads. In this way the inductance per turn is increased and, furthermore, as will be mentioned presently, the windings give zero inductance in the phantom circuit. The legends on the low pass filter giving the inductance of the series windings apply to the two opposite windmeans the inductance of the two windings in the side circuit.

In accordance with the principles that have already been stated, each end series impedance element has been adjusted so as to give 0.809-series ending and, as already explained, with thisending, the high pass wave-filter gives approximately the correct reactance annulling shunt.

Some resistance will inevitably be encountered in the inductance windings, and the computed value for the resistance R namely 76.08 ohms, includes such resistance in the inductance windings. This will generally be a small fraction, only a few 'per cent, but it will be understood that the resistances indicated in the series elements of the low pass filter in Fig. 4 reside prinings on both sides of the line, each pair of equal windings on the same core, then if the line through the low pass filter be regarded as one side of a phantom circuit, these windings will offer no impedance, and low fre quency currents on the phantom circuit will go through the low pass filter without meeting any impedance whatever. But the introduction of series resistances as shown in Fig. 4 puts resistance in each side .of the phantom circuit even thoughthe inductance windings can be ignored.

To arrive at a suitable modification to make correction in the phantom circuit for the resistances introduced in the low pass filter in Fig. 4, let each shunt condenser C of Fig. 3 be represented by two condensers in series, each of value 20 as in Fig. 4. Let thepoints between these'condensers be connected as shown in Fig. 4 with an unknown impedance at the element designated R which unknown impedance we will designate 2,. for the present.

A convenient first ste in'arriving at a proper value'for z, wil be to replace the II section. forming the mesh in which the character H is placed, by an equivalent T This has been done in Fig. 5, in which also the high pass wave-filters ave been omitted because of the high impedance of these branches at the low frequency of 135 cycles per second, in which we are at I With this understanding, the elements of the corresponding section shown in Fig: 5 can easily be worked out to get the values given by the legends on the drawings.

Fig. 5 shows the two sides of the phantom circuit with a bridge across comprising the unknown impedance 2,. recurrent network of which this makes a. mid-series phantom section, let a complete series element be Z, and let's complete shunt element be Z where Z and Z are as yet undetermined quantities. A comparison of Fig. 6 with Fig. 5.will make this apparent. By means of the values given in connection with Fig.5, the following determinations are reached:

Solving equation (12) and. making. use of the known formula for the mid-series acteristic impedance, namely.

z,:R :1'340 ohms. (15) Referring to Fig. 7, the components of the characteristic impedanceof the hantom circuit are shown by the dotted lines and the impedance Z is shown by the full lines.

The close agreement shows that the value for z, obtained above,- namely 1340 ohms, gives a very satisfactory simulation at the filters for the characteristic impedance of the phantom line over a suitable range of frequencies.

The introduction of the series resisthn'ces char-' I In the ideal Ill in the lower pass filter and the further inwas entailed thereby, lead to a moderate amount of attenuation at the filters in the phantom circuit but the magnitude of this attenuation is not serious.

I claim:

1. An electrical wave filter of the type having recurrent sections and in combination therewith apparatus to which the impedance of the wave filter is to be made to conform, each section of the wave filter having series and shunt reactance elements and each section also having added resistance ele-' ments.

2. An electrical wave filter of the type ,having recurrent sections and in combination therewith an extended transmission line, each section of the filter having series and shunt reactance elements and each section having added resistance elements whereby the impedance the filter-is matched to that of the said line over a wide frequency range.

3. A low pass electrical wave filter of the type having recurrent sections and in combination therewith an extended transmission line, eachsection'of said filter having added resistance elements whereby the said line has its impedance matched to that of the filter at lowfrequencies.

4. In combination, a transmission line,

- high pass and low pass filters connected therewith, and resistance elements introduced in series 1n the low pass filter tomatch the impedance of the two filters together'to the line at low frequencies.

5. In combination, two transmission lines constituting two side circuits and a phantom circuit, a high pass and a low pass filter in each side circuit, and resistances associated with the two low pass filters to equalize the impedance looking into the filters from the line to the impedance looking into the line from the filters, both for the side circuits and the phantom circuit.

6. In combination, a line, a high pass filter and a low pass filter in parallel in said line, a repeater associated with said high pass filter, and said low pass filter comprising resistance elements in series.

7. In combination, two lines, a high pass filter and a low pass filter in parallel in each line, a repeater associated with each high pass filter, resistances in series in each low pass filter, and a cross-conductor comprising an'esistance connecting central points of the low pass filters.

8. In combination, a line, a high pass filter and a low pass filter in parallel in said line and each having a terminal impedance value complementary to the other, said low pass filter also comprising resistance elements in series.

In testimony whereof, I have signed my name to this specification this 6th day of June, 1923.

OTTO J. ZOBEL. 

